Carbonate Dissolution in the Deep-Sea
257
a)
CO2
..
0>-----....
To
Atmosphere
From
air (Liss 1973; Millero 1979; Liss and Merlivat 1986;
Maier-Reimer and Hasselmann 1987). Only about
1 % of dissolved [C0 2 ]aqu. occurs as H 2 C0 3 , the
rest ofthe CO 2 exists in the form of different ions. E
Representatives of inorganic carbon are carbonic .5
acid, bicarbonate and carbonate. Coccolithophorids, ]
foraminifers, pteropods, and a few other organisms :;;:
build calcium carbonate shells or skeletons. Calcification can proceed both from carbonate and bicarbonate ions. In any case, it tends to drive CO 2
from the ocean to the atmosphere (Gattuso et al.
1993).
b)
c)
.Photosynthesis
Lr--_-=:::::::",,--o-< ____
Dissolution
:;:]0.16
Decay
The transfer of calcium carbonate particles from
the mixed layer to the deep ocean was introduced
as carbonate pump or alkalinity pump (Berger
1982). The vertical distribution of organic carbon
in the ocean is mainly controlled by photosynthesis, feeding, respiration, and decay (Berger et al.
1989) which contribute to the biological pump
(Revelle 1944). With respect to the water column,
Ca 2 + varies relatively little, hence the calcite saturation state is controlled by the concentration of
COt, temperature, and water pressure. Position
and thickness of the saturation horizon in the water column can be defined as the difference ACOt
between the concentration of carbonate in situ and
the concentration of saturated carbonate ion for the
mineral phase calcite (Broecker and Takahashi
1978).
Carbonate dissolution in the water column
(Culkin 1965; Edmont 1970; Murray and Riley 1971)
and at the sediment pore water interface (Santschi
et al. 1983; Le and Shackleton 1992) depend on the
disequilibrium of the total carbon dioxide content
LC0 2 • It is balanced with HC0 3 · and COt and
driven by the alkalinity (Fig. 1; Baes 1982). The
water depth in which the sea water carbonate ion
content and the concentration of carbonate ions in
equilibrium with sea water for calcite mineral phase
intercept was introduced as the hydrographic calcite lysocline (Broecker and Takahashi 1978) - also
known as "Peterson's level" (Berger 1975). It is
stated that an undersaturation of about 10 Ilmol/kg
is enough to dissolve almost all the calcite descending to the sea-floor (Broecker and Peng 1982). The
depth at which the effects of dissolution first appears in the sediments is termed sedimentary
lysocline (Berger 1975), foraminiferal lysocline
I
.Precipitation
Fig. I. ~C02-alkalinity-vector-diagram which describes
changing alkalinity and total carbon content according
to
a} supply and withdrawal of CO 2 to/from the atmosphere
b} enhanced photosynthesis or decay
c} increased CaC0 3 precipitation or dissolution
(modifiedafterBaes 1982).
(Berger 1968), or coccolith lysocline (Berger
1973a), respectively. Keir (1980) discovered that
dissolution becomes progressively more intense in
proportion to the fourth power of AC0 3 2· below the
lysocline. Where undersaturation is large enough so
that the rate of calcite sedimentation is totally compensated for by the rate of calcite dissolution, the
carbonate compensation depth (CCD) is attained
(Bramlette 1961), which is described by Archer
(1996) as the zero intercept of the %CaC0 3
versus ACOt relation.
However, the use ofthe word lysocline is rather
fuzzy; sometimes it is used in the traditional sense
to mean the shallowest depth at which dissolution
is evident, and other times it is used to mean the
257
a)
CO2
..
0>-----....
To
Atmosphere
From
air (Liss 1973; Millero 1979; Liss and Merlivat 1986;
Maier-Reimer and Hasselmann 1987). Only about
1 % of dissolved [C0 2 ]aqu. occurs as H 2 C0 3 , the
rest ofthe CO 2 exists in the form of different ions. E
Representatives of inorganic carbon are carbonic .5
acid, bicarbonate and carbonate. Coccolithophorids, ]
foraminifers, pteropods, and a few other organisms :;;:
build calcium carbonate shells or skeletons. Calcification can proceed both from carbonate and bicarbonate ions. In any case, it tends to drive CO 2
from the ocean to the atmosphere (Gattuso et al.
1993).
b)
c)
.Photosynthesis
Lr--_-=:::::::",,--o-< ____
Dissolution
:;:]0.16
Decay
The transfer of calcium carbonate particles from
the mixed layer to the deep ocean was introduced
as carbonate pump or alkalinity pump (Berger
1982). The vertical distribution of organic carbon
in the ocean is mainly controlled by photosynthesis, feeding, respiration, and decay (Berger et al.
1989) which contribute to the biological pump
(Revelle 1944). With respect to the water column,
Ca 2 + varies relatively little, hence the calcite saturation state is controlled by the concentration of
COt, temperature, and water pressure. Position
and thickness of the saturation horizon in the water column can be defined as the difference ACOt
between the concentration of carbonate in situ and
the concentration of saturated carbonate ion for the
mineral phase calcite (Broecker and Takahashi
1978).
Carbonate dissolution in the water column
(Culkin 1965; Edmont 1970; Murray and Riley 1971)
and at the sediment pore water interface (Santschi
et al. 1983; Le and Shackleton 1992) depend on the
disequilibrium of the total carbon dioxide content
LC0 2 • It is balanced with HC0 3 · and COt and
driven by the alkalinity (Fig. 1; Baes 1982). The
water depth in which the sea water carbonate ion
content and the concentration of carbonate ions in
equilibrium with sea water for calcite mineral phase
intercept was introduced as the hydrographic calcite lysocline (Broecker and Takahashi 1978) - also
known as "Peterson's level" (Berger 1975). It is
stated that an undersaturation of about 10 Ilmol/kg
is enough to dissolve almost all the calcite descending to the sea-floor (Broecker and Peng 1982). The
depth at which the effects of dissolution first appears in the sediments is termed sedimentary
lysocline (Berger 1975), foraminiferal lysocline
I
.Precipitation
Fig. I. ~C02-alkalinity-vector-diagram which describes
changing alkalinity and total carbon content according
to
a} supply and withdrawal of CO 2 to/from the atmosphere
b} enhanced photosynthesis or decay
c} increased CaC0 3 precipitation or dissolution
(modifiedafterBaes 1982).
(Berger 1968), or coccolith lysocline (Berger
1973a), respectively. Keir (1980) discovered that
dissolution becomes progressively more intense in
proportion to the fourth power of AC0 3 2· below the
lysocline. Where undersaturation is large enough so
that the rate of calcite sedimentation is totally compensated for by the rate of calcite dissolution, the
carbonate compensation depth (CCD) is attained
(Bramlette 1961), which is described by Archer
(1996) as the zero intercept of the %CaC0 3
versus ACOt relation.
However, the use ofthe word lysocline is rather
fuzzy; sometimes it is used in the traditional sense
to mean the shallowest depth at which dissolution
is evident, and other times it is used to mean the